Packet decoding for H-ARQ transmission

ABSTRACT

Techniques for efficiently decoding packets sent with H-ARQ are described. Packet decoding for H-ARQ may be performed based on local search around a start of packet (SOP) decision for a packet. The SOP decision for the packet may be made based on traffic detection results for received transmissions. At least one SOP hypothesis may be determined for the packet based on the SOP decision, and the received transmissions may be decoded based on the at least one SOP hypothesis. A sliding SOP window may be used to keep track of SOP hypotheses for the packet. The sliding window may be initialized at an earliest received transmission, moved forward for each subsequent received transmission with no detected packet data, and maintained at the first received transmission with detected traffic. Rotating buffers may be used to store received transmissions for packets for decoding.

BACKGROUND

I. Field

The present disclosure relates generally to communication, and morespecifically to techniques for decoding data in a wireless communicationsystem.

II. Background

In a wireless communication system, a transmitter typically processes(e.g., encodes and symbol maps) a packet of data to generate datasymbols, which are modulation symbols for data. The transmitter furtherprocesses the data symbols to generate a modulated signal and transmitsthis signal via a wireless channel. The wireless channel distorts thetransmitted signal with a channel response and further degrades thesignal with noise and interference. A receiver receives the transmittedsignal and processes the received signal to obtain data symbolestimates, which are estimates of the transmitted data symbols. Thereceiver then processes (e.g., demodulates and decodes) the data symbolestimates to obtain a decoded packet.

The system may employ hybrid automatic repeat request (H-ARQ)transmission to improve reliability for data transmission. With H-ARQ,the transmitter may send one or more transmissions for a packet, onetransmission at a time. The receiver may receive each transmission sentby the transmitter and may attempt to decode different combinations ofreceived transmissions to recover the packet. The receiver may send anacknowledgement (ACK) when the packet is decoded correctly, and thetransmitter may send a transmission for a new packet upon receiving theACK.

Packet decoding for H-ARQ transmission may be challenging for severalreasons. First, the transmitter may send transmissions for packets in adiscontinuous manner so that the receiver may not know with certaintywhether or not a transmission for a packet has been received in a givenframe. Second, the transmissions sent by the transmitter may not includeinformation to indicate the first transmission (or start) of eachpacket. The receiver may then attempt to decode the receivedtransmissions for various hypotheses, with each hypothesis correspondingto a different guess as to when a given packet was first sent. Decodingfor many hypotheses may increase decoder complexity and/or require morebuffering.

There is therefore a need in the art for techniques to efficientlydecode data for H-ARQ.

SUMMARY

Techniques for efficiently decoding packets sent with H-ARQ aredescribed herein. These techniques may be able to handle multiple startof packet (SOP) hypotheses for a packet as well as delayed and timechanging SOP hypotheses for the packet in an efficient manner.

In an aspect, packet decoding for H-ARQ is performed based on a localsearch around a SOP decision for a packet. A plurality of transmissionsmay be received in a plurality of frames for the packet, e.g., onetransmission in each frame. Traffic detection may be performed todetermine whether each received transmission carry packet data or nopacket data. A SOP decision, which is a decision of a frame in which thepacket might have started, may be made based on the traffic detectionresults for the plurality of received transmissions. At least one SOPhypothesis may be determined for the packet based on the SOP decision.Each SOP hypothesis may correspond to a different frame hypothesized tobe the SOP of the packet. For example, one SOP hypothesis may cover aframe prior to the SOP decision, another SOP hypothesis may cover theframe for the SOP decision, and yet another SOP hypothesis may cover aframe after the SOP decision. The plurality of received transmissionsmay be decoded based on the at least one SOP hypothesis for the packet.

In another aspect, a sliding SOP window is used to keep track of SOPhypotheses for a packet. The sliding window may cover at least one SOPhypothesis for the packet and may be determined, e.g., based on trafficdetection results for a plurality of transmissions received for thepacket. For example, the sliding window may be initialized at anearliest received transmission, moved forward for each subsequentreceived transmission with no detected packet data, and maintained atthe first received transmission with detected traffic. Transmissionsreceived prior to the first received transmission with detected packetdata may be stored. Decoding may be started when the first receivedtransmission with packet data is detected. The plurality of receivedtransmissions may be decoded based on the at least one SOP hypothesisfor the packet. In each frame after decoding is started, all SOPhypotheses applicable for that frame may be determined, and decoding maybe performed for one SOP hypothesis at a time.

In yet another aspect, rotating buffers are used to store receivedtransmissions for packets for decoding. At least one SOP hypothesis maybe determined for a packet. At least one buffer among a plurality ofbuffers may be assigned to the at least one SOP hypothesis for thepacket, e.g., one buffer for each SOP hypothesis. The plurality ofbuffers may be selected for assignment to SOP hypotheses in apredetermined order, e.g., in a sequential and circular manner. Eachbuffer may store information, e.g., log-likelihood ratios (LLRs), for atleast one received transmission for the SOP hypothesis to which thebuffer is assigned.

Various aspects and features of the disclosure are described in furtherdetail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates H-ARQ transmission.

FIG. 2 shows packet decoding for H-ARQ without SOP search.

FIG. 3 shows packet decoding for H-ARQ with global SOP search.

FIG. 4 shows packet decoding for H-ARQ with local SOP search.

FIGS. 5A and 5B show packet decoding for H-ARQ with 3-frame and 5-framesliding SOP windows, respectively.

FIGS. 6A and 6B show packet decoding for H-ARQ with three and fiverotating buffers, respectively, for 3-frame and 5-frame sliding SOPwindows.

FIG. 7 shows storage of received transmissions in three buffers.

FIG. 8 shows a process for decoding packets sent with H-ARQ.

FIG. 9 shows an apparatus for decoding packets sent with H-ARQ.

FIG. 10 shows a process for decoding packets using rotating buffers.

FIG. 11 shows an apparatus for decoding packets using rotating buffers.

FIG. 12 shows a block diagram of a base station and a terminal.

FIG. 13 shows a block diagram of a receive (RX) data processor.

FIG. 14 shows a state diagram for LLR computation and decoding.

DETAILED DESCRIPTION

The techniques described herein may be used for various systems thatemploy H-ARQ transmission, which may also be referred to as incrementalredundancy (IR) transmission. With H-ARQ, a transmitter sends one ormore transmissions for a packet until the packet is decoded correctly bya receiver or the maximum number of transmissions has been sent. H-ARQimproves reliability for data transmission and supports rate adaptationfor packets in the presence of changes in channel conditions.

FIG. 1 illustrates H-ARQ transmission. A transmitter processes (e.g.,encodes and symbol maps) a packet of data (Packet 1) and generates datasymbols. The transmitter sends a first transmission (Trans 1) for Packet1 in frame m. In general, a frame may cover any time duration and may beof different durations in different systems. A receiver receives andprocesses (e.g., demodulates and decodes) the first transmission,determines that Packet 1 is decoded in error, and sends a negativeacknowledgement (NAK) to the transmitter in frame m+q. The transmitterreceives the NAK and sends a second transmission (Trans 2) for Packet 1in frame m+Q. Each transmission may contain different information forthe packet. The receiver receives the second transmission, processes thefirst and second transmissions, determines that Packet 1 is decodedcorrectly, and sends an ACK in frame m+Q+q. The transmitter receives theACK and terminates the transmission of Packet 1. The transmitter thenprocesses the next packet (Packet 2) and sends transmissions for Packet2 in similar manner.

In FIG. 1, transmissions for packets are sent every Q frames. To improvechannel utilization, the transmitter may send up to Q packets in aninterlaced manner. A first interlace may be defined for frames m, m+Q,etc., a second interlace may be defined for frames m+1, m+Q+1, etc., anda Q-th interlace may be defined for frames m+Q−1, m+2Q−1, etc. The Qinterlaces may be offset from one another by one frame. The transmittermay send up to Q packets on the Q interlaces. For example, if Q=2, thenthe first interlace may include odd-numbered frames, and the secondinterlace may include even-numbered frames. In general, the H-ARQretransmission delay Q and the ACK/NAK delay q may be selected toprovide sufficient processing time for both the transmitter andreceiver.

For clarity, FIG. 1 shows transmission of both NAKs and ACKs. For anACK-based scheme, an ACK is sent if a packet is decoded correctly, andNAKs are not sent and are presumed by the absence of ACKs.

H-ARQ makes efficient use of a communication channel by having thetransmitter and receiver work together to occupy the channel just longenough for each packet to be decoded correctly. The transmitter sendsone transmission at a time for a packet until the packet is decodedcorrectly and an ACK is received. The receiver processes alltransmissions received for the packet, with each transmission providingadditional information for the packet. This ability to process alltransmissions received for the packet assumes that the receiver canascertain the start of packet (SOP).

The system may support discontinuous traffic transmission (DTTX), e.g.,for sticky assignment of radio resources. With sticky assignment,signaling may not be sent explicitly to indicate whether previouslyassigned radio resources are still assigned or being used. For DTTX, thetransmitter may or may not send a traffic transmission, which is atransmission for a packet, in a given frame. The transmitter may sendnothing and/or signaling in a DTTX frame, which is a frame with notraffic transmission. For example, signaling may be sent in a DTTX frameto maintain a traffic channel for the receiver with a sticky assignment.

FIG. 2 shows an example of packet decoding for H-ARQ without SOP search.In this example, no information is sent with a traffic transmission toindicate which transmission is being sent among all possibletransmissions for a packet. The receiver may perform traffic detectionin each frame to determine whether or not a traffic transmission hasbeen received in that frame. Traffic detection may also be referred toas erasure detection, packet data detection, DTX detection, etc. Thetraffic detection may provide a “fail” indication if no traffictransmission is detected or a “pass” indication if traffic transmissionis detected. However, the fail/pass indications may be erroneous. Amissed detection occurs when a traffic transmission was sent but thetraffic detection provides a “fail” indication. A false alarm occurswhen a traffic transmission was not sent but the traffic detectionprovides a “pass” indication.

In FIG. 2, the traffic detection is assumed to be accurate in eachframe. The receiver receives the first transmission (T1) for packet 1(Pkt 1) in frame F₁ and decodes packet 1 in error based solely on thistransmission. The receiver receives the second transmission (T2) forpacket 1 in frame F₂ and decodes packet 1 correctly based on the firstand second transmissions. The receiver receives the first transmissionfor packet 2 (Pkt 2) in frame F₃ and decodes packet 2 in error basedsolely on the first transmission. The receiver receives the secondtransmission for packet 2 in frame F₄ and decodes packet 2 correctlybased on the first and second transmissions. The receiver receives atransmission for packet 3 (Pkt 3) in each of frames F₅ through F₈,decodes packet 3 in error in each of frames F₅ through F₇, and decodespacket 3 correctly in frame F₈ based on the four transmissions receivedfor packet 3.

No transmissions are sent in frames F₉, F₁₀ and F₁₁, the trafficdetection provides “fail” indications, and the receiver does not attemptdecoding in any of these frames. The receiver receives a transmissionfor packet 4 (Pkt 4) in each of frames F₁₂ through F₁₅, decodes packet 4in error in each of frames F₁₂ through F₁₄, and decodes packet 4correctly in frame F₁₅ based on the four transmissions received forpacket 4. The receiver receives a transmission for packet 5 (Pkt 5) ineach of frames F₁₆ through F₁₈, decodes packet 5 in error in each offrames F₁₆ and F₁₇, and decodes packet 5 correctly in frame F₁₈ based onthe three transmissions received for packet 5. No transmissions are sentin frames F₁₉ and F₂₀, the traffic detection provides “fail”indications, and the receiver does not attempt decoding in any of theseframes. The receiver receives a transmission for packet 6 (Pkt 6) ineach of frames F₂₁ and F₂₂, decodes packet 6 in error in frame F₂₁, anddecodes packet 6 correctly in frame F₂₂ based on the two transmissionsreceived for packet 6.

FIG. 2 shows an ideal scenario in which the receiver is able tocorrectly determine the SOP of each packet based on totally reliabletraffic detection. In this scenario, the receiver is able to make acorrect SOP decision for each packet. In the example shown in FIG. 2,the SOP decision for packet 1 is frame F₁, the SOP decision for packet 2is frame F₃, the SOP decision for packet 3 is frame F₅, etc. Withcorrect SOP decisions, the receiver is able to process each packet assoon as possible and with minimum decoding complexity based on the SOPdecision for that packet. In each frame, the receiver may attempt todecode the current packet based on all transmissions received for thatpacket. In FIG. 2, the receiver performs a total of 17 decoding attemptsin 23 frames and performs at most one decoding attempt per frame.

The traffic detection may not be totally reliable, and some errors intraffic detection may be made. Consequently, the SOP decisions forpackets may be incorrect. For example, a false alarm may result fromerroneous detection of a traffic transmission when none was sent, whichmay then result in a SOP decision for a packet being at a wrong frame.The incorrect SOP decision may then result in decoding failure in eachsubsequent frame. Potential SOP decision errors may be addressed invarious manners, as described below.

FIG. 3 shows an example of packet decoding for H-ARQ with global SOPsearch. With global SOP search, each frame is treated as a potential SOPregardless of the traffic detection for that frame. A given packet maythus be decoded based on one or more SOP hypotheses. Each SOP hypothesiscorresponds to a different frame hypothesized to be the SOP of thepacket. For each SOP hypothesis, decoding may be attempted in eachframe, starting with the hypothesized SOP frame, until the packet isdecoded correctly. The sequence of transmissions in FIG. 3 is the sameas in FIG. 2.

In frame F₀, decoding may be attempted for one SOP hypothesis for packet1, which is a first SOP hypothesis H11 corresponding to the SOP ofpacket 1 being at frame F₀. The decoding of the transmission received inframe F₀ for the first SOP hypothesis fails. In frame F₁, decoding maybe attempted for two SOP hypotheses for packet 1, which includes thefirst SOP hypothesis as well as a second SOP hypothesis H12corresponding to the SOP of packet 1 being at frame F₁. The decoding ofthe transmissions received in frames F₀ and F₁ for the first SOPhypothesis fails, and the decoding of the transmission received in frameF₁ for the second SOP hypothesis also fails. In frame F₂, decoding maybe attempted for three SOP hypotheses for packet 1, which includes thefirst and second SOP hypotheses as well as a third SOP hypothesis H13corresponding to the SOP of packet 1 being at frame F₂. The decoding ofthe transmissions received in frames F₀, F₁ and F₂ for the first SOPhypothesis fails, the decoding of the transmissions received in framesF₁ and F₂ for the second SOP hypothesis succeeds, and the decoding ofthe transmission received in frame F₂ for the third SOP hypothesis alsofails.

In frame F₃, one decoding attempt may be made for a first SOP hypothesisH21 for packet 2, which corresponds to the SOP of packet 2 being atframe F₃. In frame F₄, two decoding attempts may be made, one for thefirst SOP hypothesis (which succeeds) and another for a second SOPhypothesis H22 corresponding to the SOP of packet 2 being at frame F₄(which fails).

For packet 3, one decoding attempt may be made in frame F₅ for a firstSOP hypothesis H31 corresponding to the SOP of packet 3 being at frameF₅. Two decoding attempts may be made in frame F₆ for the first SOPhypothesis as well as a second SOP hypothesis H32 corresponding for theSOP of packet 3 being at frame F₆. Three decoding attempts may be madein frame F₇ for the first and second SOP hypotheses as well as a thirdSOP hypothesis H33 corresponding to the SOP of packet 3 being at frameF₇. Four decoding attempts may be made in frame F₈ for the first, secondand third SOP hypotheses as well as a fourth SOP hypothesis H34corresponding to the SOP of packet 3 being at frame F₈.

The decoding for each subsequent packet may proceed in similar manner.For packet 4, decoding attempts may be made for seven SOP hypotheses H41through H47 corresponding to the SOP of packet 4 being at frames F₉through F₁₅, respectively. The number of decoding attempts in each frameis given in FIG. 3. In this example, up to six transmissions may be sentfor a given packet. Thus, in frame F₁₅, decoding is not attempted forthe first SOP hypothesis H41, which would have terminated in frame F₁₄.

In the example shown in FIG. 3, up to 62 decoding attempts may be madeover 23 frames, and up to six decoding attempts may be made per frame.The number of decoding attempts may increase significantly when thereare many SOP hypotheses, e.g., due to no traffic transmissions in framesF₉, F₁₀ and F₁₁ prior to the first transmission for packet 4 in frameF₁₂. Global SOP search may ensure that all packets can be decoded,albeit at the expense of higher decoding complexity. In general, it isdesirable to reduce the number of SOP hypotheses while achievingreliable H-ARQ packet decoding.

In an aspect, packet decoding for H-ARQ is performed based on localsearch around a SOP decision for a packet. If the SOP decision is closeto the true SOP of the packet, then the number of decoding attempts maybe reduced by performing local search around the SOP decision.

FIG. 4 shows an example of packet decoding for H-ARQ with local SOPsearch. In the example shown in FIG. 4, the true SOP is assumed to be atmost one frame away from the SOP decision. For each packet, a SOP windowcovering up to three SOP hypotheses may be centered on the SOP decision,and decoding attempts may be made for all SOP hypotheses within the SOPwindow. The sequence of transmissions in FIG. 4 is the same as in FIG.2.

The SOP decision for packet 1 is frame F₁, which is the first frame inwhich the traffic detection provides a “pass” indication. Decodingattempts may be made for packet 1 for three SOP hypotheses of F₀, F₁ andF₂, which are within one frame of the SOP decision. The SOP decision forpacket 2 is frame F₃. Decoding attempts may be made for packet 2 for twoSOP hypotheses of F₃ and F₄ (and not F₂, which is known to carry packet1). The SOP decision for packet 3 is frame F₅. Decoding attempts may bemade for packet 3 for two SOP hypotheses of F₅ and F₆ (and not F₄, whichis known to carry packet 2). Frames F₇ and F₈ are outside of the SOPwindow for packet 3, and no decoding attempts are made for SOPhypotheses with these frames. The SOP decision for packet 4 is frameF₁₂, which is the first frame in which the traffic detection provides a“pass” indication after three frames of no traffic transmissions.Decoding attempts may be made for packet 4 for three SOP hypotheses ofF₁₁, F₁₂ and F₁₃, and no decoding attempts are made for SOP hypothesesof F₁₄ and F₁₅, which are outside of the SOP window for packet 4. Thedecoding for each subsequent packet may proceed in similar manner.

In the example shown in FIG. 4, up to 39 decoding attempts may be madeover 23 frames, and up to three decoding attempts may be made per framefor up to three SOP hypotheses within a 3-frame SOP window. The numberof decoding attempts in each frame is indicated by the “decoding count”entry for that frame. The total number of decoding attempts may beobtained by accumulating the decoding count values over 23 frames. SOPwindows of other sizes (e.g., two, four, five, or more frames) may alsobe used, possibly at the expense of higher decoding complexity. The SOPwindow may also be centered at another frame instead of the SOPdecision.

The local SOP search relies on the true SOP being close to the SOPdecision and also within the SOP window. Packet decoding performance maydegrade when the SOP decision is not sufficiently close to the true SOP,which may result from missed detection or false alarm. A history oftraffic detection indications may be stored and used to detect forpossible false alarms. For example, a single-frame transmission may beidentified whenever the traffic detection provides “fail”, “pass”, and“fail” indications (or “pass”, “fail”, and “fail” indications) for threeconsecutive frames. This single-frame transmission may be declared as afalse alarm if it is decoded in error. In one design, all SOP hypothesesassociated with the false alarm are eliminated, and no decoding attemptsare made for these SOP hypotheses. In another design, the SOP hypothesesfor the false alarm are decoded last in each frame.

In another aspect, a sliding SOP window is used to keep track of SOPhypotheses for a packet. The sliding SOP window may be moved forward ineach frame until the traffic detection provides a “pass” indication anda SOP decision is made for the packet. Transmissions received in framescovered by the sliding SOP window may be prepared for decoding as thewindow is moved forward. However, decoding may be delayed until the SOPdecision is made. The use of the sliding SOP window may reduce decodingcomplexity while providing reliable packet decoding for H-ARQ.

FIG. 5A shows an example of packet decoding for H-ARQ with a 3-framesliding SOP window. The sequence of transmissions in FIG. 5A is the sameas in FIG. 2. Initially, a Reset command is received to reset thesliding SOP window for packet 1. In one design, the sliding SOP windowfor a given packet is centered at the SOP decision for that packet andmay include one frame to the left of the SOP decision if it is not aframe for another correctly decoded packet and may also include oneframe to the right of the SOP decision if a traffic transmission isreceived in this frame.

For packet 1, the traffic detection provides a “fail” indication inframe F₀, and the SOP window for packet 1 is initially centered at thisframe. The transmission received in frame F₀ may be stored for the firstSOP hypothesis H11 for packet 1 instead of decoded. A receivedtransmission may also be processed prior to storage in a buffer. Theprocessing may include computing LLRs for code bits in the receivedtransmission, combining the LLRs for the current frame with LLRs forprior frames (if any) for the current packet, etc. In frame F₁, thetraffic detection provides a “pass” indication, the SOP decision forpacket 1 is frame F₁, and the sliding SOP window moves over by one frameand remains. The transmissions received in frames F₀ and F₁ may becombined and decoded (in error) for the first SOP hypothesis. Thetransmission received in frame F₁ may also be decoded (in error) for thesecond SOP hypothesis H12 for packet 1. In frame F₂, the right side ofthe SOP window expands by one frame and remains. The transmissionsreceived in frames F₀, F₁ and F₂ may be combined and decoded (in error)for the first SOP hypothesis. The transmissions received in frames F₁and F₂ may also be combined and decoded (correctly) for the second SOPhypothesis. The transmission received in frame F₂ may be decoded (inerror) for the third SOP hypothesis H13 for packet 1. The SOP windowcovers three SOP hypotheses for packet 1.

For packet 2, the traffic detection provides a “pass” indication inframe F₃, the SOP decision for packet 2 is frame F₃, and the SOP windowfor packet 2 is centered at frame F₃. The left side of the SOP window isnot expanded since frame F₂ is known to carry packet 1. The transmissionreceived in frame F₃ may be decoded (in error) for the first SOPhypothesis H21 for packet 2. In frame F₄, the right side of the SOPwindow is expanded by one frame. The transmissions received in frames F₃and F₄ may be combined and decoded (correctly) for the first SOPhypothesis, and the transmission received in frame F₄ may be decoded (inerror) for the second SOP hypothesis H22 for packet 2. The SOP windowcovers two SOP hypotheses for packet 2.

For packet 3, the traffic detection provides a “pass” indication inframe F₅, the SOP decision for packet 3 is frame F₅, and the SOP windowfor packet 3 is centered at frame F₅. The transmission received in frameF₅ may be decoded (in error) for the first SOP hypothesis H31 for packet3. In frame F₆, the right side of the SOP window is expanded by oneframe to cover the second SOP hypothesis H32 for packet 3. Decoding maybe attempted for the two SOP hypotheses for packet 3 in each of framesF₆, F₇ and F₈.

For packet 4, the traffic detection provides a “fail” indication in eachof frames F₉, F₁₀ and F₁₁ and a “pass” indication in frame F₁₂. The SOPdecision for packet 4 is frame F₁₂. The sliding SOP window for packet 4moves over by one frame in each of frames F₁₀, F₁₁ and F₁₂, remains atframe F₁₂, and is expanded on the right side by one frame in frame F₁₃.The transmissions received in frame F₉, F₁₀, F₁₁, F₁₂ and F₁₃ may bestored for up to three SOP hypotheses in each frame. In particular, theSOP window covers SOP hypothesis H41 for packet 4 in frame F₉, SOPhypotheses H41 and H42 in frame F₁₀, SOP hypotheses H42 and H43 in frameF₁₁, SOP hypotheses H43 and H44 in frame F₁₂, and SOP hypotheses H43,H44 and H45 in frame F₁₃. Decoding may be attempted for two SOPhypotheses H43 and H44 in frames F₁₂, and for three SOP hypotheses H43,H44 and H45 in each of frames F₁₃, F₁₄ and F₁₅. The decoding for packets5 and 6 may proceed in similar manner.

In the example shown in FIG. 5A, up to 36 decoding attempts may be madeover 23 frames, and up to three decoding attempts may be made per framein the worst case. The total number of decoding attempts may be obtainedby accumulating the decoding count values over 23 frames. The number ofdecoding attempts for the sliding SOP window may be fewer than thenumber of decoding attempts for the local SOP search.

FIG. 5B shows an example of packet decoding for H-ARQ with a 5-framesliding SOP window. The sequence of transmissions in FIG. 5B is the sameas in FIG. 2. In this design, the sliding SOP window for each packet iscentered at the SOP decision for that packet and may include up to twoframes to the left and/or up to two frames to the right of the SOPdecision.

For packet 1, the traffic detection provides a “pass” indication inframe F₁, the SOP decision is frame F₁, and the SOP window is centeredat frame F₁ and includes three SOP hypotheses for F₀, F₁ and F₂. Forpacket 2, the SOP decision is frame F₃, and the SOP window is centeredat frame F₃ and includes two SOP hypotheses for F₃ and F₄. For packet 3,the SOP decision is frame F₅, and the SOP window is centered at frame F₅and includes three SOP hypotheses for F₅, F₆ and F₇. For packet 4, theSOP window is initially centered at frame F₉, then moves over by oneframe in each of frames F₁₀, F₁₁ and F₁₂, remains at frame F₁₂ where theSOP decision is made, and is expanded on the right side to cover framesF₁₃ and F₁₄. The SOP window for packets 5 and 6 may be determined insimilar manner. In the example shown in FIG. 5B, up to 47 decodingattempts may be made over 23 frames, and up to five decoding attemptsmay be made per frame in the worst case.

In the examples shown in FIGS. 5A and 5B, after successfully decodingpacket 3, the received transmissions may be processed (but notnecessarily decoded) in anticipation of a SOP decision for packet 4. Thereceived transmissions may be stored for up to K SOP hypotheses as newtransmissions continue to arrive. K may be defined as K=┌N/2┐, where Nis the width of the sliding SOP window and “┌ ┐” is a ceiling operator.In FIG. 5A, N=3, K=2, the 3-frame sliding SOP window is centered at theSOP decision, and the received transmissions may be stored for up to twoSOP hypotheses within the SOP window before a SOP decision is made. InFIG. 5B, N=5, K=3, the 5-frame sliding SOP window is centered at the SOPdecision, and the received transmissions may be stored for up to threeSOP hypotheses within the SOP window before a SOP decision is made.Regardless of the sliding SOP window size, earlier SOP hypotheses thatfall outside of the sliding SOP window before any SOP decision is made(e.g., SOP hypotheses H41 and H42 for packet 4 in FIG. 5A) may bedropped without attempting decoding on these SOP hypotheses. When theSOP decision is made, the SOP window stops sliding, and decoding may beattempted for all SOP hypotheses within the SOP window until successfuldecoding is declared or the maximum number of transmissions has elapsed.

In yet another aspect, rotating buffers are used to store informationfor packets for decoding. N buffers may be used to store information forup to N SOP hypotheses for a given packet, one buffer per SOPhypothesis, where N may be determined by the SOP window and/or otherfactors. Whenever a new SOP hypothesis is formed, one of the N buffersmay be assigned to store information for this SOP hypothesis. The Nbuffers may be selected in a predetermined order and assigned to new SOPhypotheses.

FIG. 6A shows an example of packet decoding for H-ARQ with threerotating buffers for a 3-frame sliding SOP window. In this example,buffers 1, 2 and 3 are assigned in a sequential and circular manner tonew SOP hypotheses. The sequence of transmissions in FIG. 6A is the sameas in FIG. 2.

For packet 1, the first SOP hypothesis H11 is declared in frame F₀ andassigned to buffer 1, the second SOP hypothesis H12 is declared in frameF₁ and assigned to buffer 2, and the third SOP hypothesis H13 isdeclared in frame F₂ and assigned to buffer 3. Each buffer storesinformation for all received transmissions for its assigned SOPhypothesis, as described below. For packet 2, the first SOP hypothesisH21 is declared in frame F₃ and assigned to buffer 1 (which is theoldest previously assigned buffer at frame F₃), and the second SOPhypothesis H22 is declared in frame F₄ and assigned to buffer 2. Forpacket 3, the first SOP hypothesis H31 is assigned to buffer 3 (which isthe oldest previously assigned buffer at frame F₅), and the second SOPhypothesis H32 is assigned to buffer 1. For packet 4, the first SOPhypothesis H41 is assigned to buffer 2 (which is the oldest previouslyassigned buffer at frame F₉), the second SOP hypothesis H42 is assignedto buffer 3, the third SOP hypothesis H43 is assigned to buffer 1, thefourth SOP hypothesis H44 is assigned to buffer 2, and the fifth SOPhypothesis H45 is assigned to buffer 3. Buffers may be assigned to theSOP hypotheses for packets 5 and 6 in similar manner.

FIG. 7 shows an example of how buffers 1, 2 and 3 may store informationfor the first, second, and third SOP hypotheses, respectively, forpacket 1 in FIG. 6A. Up to six transmissions T1 through T6 may be sentfor packet 1, one transmission at a time, starting with transmission T1.Two transmissions T1 and T2 are sent in frames F₁ and F₂, respectively.Three transmissions R0, R1 and R2 are received in frames F₀, F₁ and F₂,respectively. Buffer 1 stores the three received transmissions R0, R1and R2 under an assumption that the SOP of packet 1 is at frame F₀,which is incorrect, and thus R0, R1 and R2 are stored in the wronglocations in buffer 1. Buffer 2 stores the two received transmissions R1and R2 under an assumption that the SOP of packet 1 is at frame F₁,which is correct, and thus R1 and R2 are stored in the proper locationsin buffer 2. Buffer 3 stores the received transmission R2 under anassumption that the SOP of packet 1 is at frame F₂, which is alsoincorrect, and thus is stored in the wrong location in buffer 3. Theinformation in buffer 2 may be decoded correctly whereas the informationin buffers 1 and 3 may be decoded in error.

In general, the received transmissions may be stored directly in thebuffers or may be processed prior to storage. For example, LLRs may becomputed and stored in the buffers. Information for differenttransmissions may be stored in different locations of a buffer (e.g., asshown in FIG. 7) and/or may be combined with other information in thebuffer (not shown in FIG. 7).

FIG. 6B shows an example of packet decoding for H-ARQ with five rotatingbuffers for a 5-frame sliding SOP window. In this example, buffers 1through 5 are assigned in a sequential and circular manner to new SOPhypotheses. The sequence of transmissions in FIG. 6B is the same as inFIG. 2.

For packet 1, the first SOP hypothesis of F₀ is assigned to buffer 1,the second SOP hypothesis of F₁ is assigned to buffer 2, and the thirdSOP hypothesis of F₂ is assigned to buffer 3. For packet 2, the firstSOP hypothesis of F₃ is assigned to buffer 4, and the second SOPhypothesis of F₄ is assigned to buffer 5. For packet 3, the first SOPhypothesis of F₅ is assigned to buffer 1 (which is the oldest previouslyassigned buffer at frame F₅), the second SOP hypothesis of F₆ isassigned to buffer 2, and the third SOP hypothesis of F₇ is assigned tobuffer 3. For packet 4, the first SOP hypothesis of F₉ is assigned tobuffer 4 (which is the oldest previously assigned buffer at frame F₉),the second SOP hypothesis of F₁₀ is assigned to buffer 5, the third SOPhypothesis of F₁₁ is assigned to buffer 1, the fourth SOP hypothesis ofF₁₂ is assigned to buffer 2, the fifth SOP hypothesis of F₁₃ is assignedto buffer 3, and the sixth SOP hypothesis of F₁₄ is assigned to buffer4. Buffers may be assigned to the SOP hypotheses for packets 5 and 6 insimilar manner.

FIGS. 4 through 6B show some example SOP window sizes. In general, theSOP window size may be selected based on various factors such as thereliability of the traffic detection (e.g., smaller SOP window for morereliable traffic detection), the maximum number of transmissions for apacket, decoding complexity, etc.

FIG. 8 shows a process 800 for decoding packets sent with H-ARQ. Aplurality of transmissions may be received in a plurality of frames fora packet, e.g., one transmission in each frame (block 812). Theplurality of received transmissions may include zero, one, or multipletransmissions without packet data (or traffic) and at least onetransmission with packet data. Whether each received transmission carrypacket data or no packet data may be detected (block 814).

A sliding window indicative of at least one SOP hypothesis for thepacket may be determined, e.g., based on detection results for theplurality of received transmissions (block 816). For example, thesliding window may be initialized at an earliest received transmissionamong the plurality of received transmissions, moved forward for eachsubsequent received transmission with no detected packet data, andmaintained at the first received transmission with detected packet data.Each SOP hypothesis may correspond to a different frame (or receivedtransmission) hypothesized to be the SOP of the packet. The slidingwindow may include (a) up to N SOP hypotheses for the packet, (b) up toL SOP hypotheses for up to L received transmissions with no detectedpacket data prior to the first received transmission with detectedpacket data, and (c) up to R SOP hypotheses for up to R receivedtransmissions after the first received transmission with detected packetdata, where N, L and R may be any suitable integer values. For example,N may be equal to three, and L and R may each be equal to one.

The plurality of received transmissions may be decoded based on the atleast one SOP hypothesis for the packet (block 818). Transmissionsreceived prior to the first received transmission with detected packetdata may be stored. Decoding may be started when the first receivedtransmission with packet data is detected. In each frame after decodingis started, one or more SOP hypotheses applicable for that frame may bedetermined, and decoding may be performed for one SOP hypothesis at atime. The one or more SOP hypotheses applicable for each frame have thehypothesized SOP of the packet being at that frame or earlier. Thedecoding for each SOP hypothesis may be based on all transmissionsreceived for that SOP hypothesis in that frame or earlier.

FIG. 9 shows an apparatus 900 for decoding packets sent with H-ARQ.Apparatus 900 includes means for receiving a plurality of transmissionsin a plurality of frames for a packet, e.g., one transmission in eachframe (module 912), means for detecting whether each receivedtransmission carry packet data or no packet data (module 914), means fordetermining a sliding window indicative of at least one SOP hypothesisfor the packet, e.g., based on detection results for the plurality ofreceived transmissions (module 916), and means for decoding theplurality of received transmissions based on the at least one SOPhypothesis for the packet (module 918). Modules 912 to 918 may compriseprocessors, electronics devices, hardware devices, electronicscomponents, logical circuits, memories, etc., or any combinationthereof.

FIG. 10 shows a process 1000 for using rotating buffers to decodepackets sent with H-ARQ. At least one SOP hypothesis may be determinedfor a packet (block 1012). At least one buffer among a plurality ofbuffers may be assigned to the at least one SOP hypothesis for thepacket, one buffer for each SOP hypothesis (block 1014). The pluralityof buffers may be selected for assignment to SOP hypotheses in apredetermined order, e.g., in a sequential and circular manner. Thenumber of buffers may be equal to the maximum number of possible SOPhypotheses for the packet. At least one received transmission for eachSOP hypothesis may be stored in the buffer assigned to that SOPhypothesis (block 1016). For example, each buffer may store LLRs for theat least one received transmission for its SOP hypothesis. A SOPhypothesis for a second packet may be determined (block 1018). A nextbuffer may then be assigned to the SOP hypothesis for the second packet(block 1018). The next buffer may be the buffer after the at least onebuffer assigned to the prior packet and among the plurality of buffers

FIG. 11 shows an apparatus 1100 for using rotating buffers to decodepackets sent with H-ARQ. Apparatus 1100 includes means for determiningat least one SOP hypothesis for a packet (module 1112), means forassigning at least one buffer among a plurality of buffers to the atleast one SOP hypothesis for the packet, one buffer for each SOPhypothesis (module 1114), means for storing at least one receivedtransmission for each SOP hypothesis in a buffer assigned to the SOPhypothesis (module 1116), means for determining a SOP hypothesis for asecond packet (module 1118), and means for assigning a next buffer tothe SOP hypothesis for the second packet, with the next buffer beingafter the at least one buffer and among the plurality of buffers (module1120). Modules 1112 to 1120 may comprise processors, electronicsdevices, hardware devices, electronics components, logical circuits,memories, etc., or any combination thereof.

FIG. 12 shows a block diagram of a design of a base station 1200 and aterminal 1250. In this design, base station 1200 and terminal 1250 areeach equipped with a single antenna. For forward link transmission, atbase station 1200, a transmit (TX) data and signaling processor 1210receives packets of data for one or more terminals from a data source1208, processes (e.g., formats, encodes, interleaves, and symbol maps)each packet, and provides a set of data symbols for each packet.Processor 1210 may provide a different subset of data symbols for eachtransmission of a given packet sent with H-ARQ. Processor 1210 alsoreceives signaling (e.g., ACKs/NAKs for H-ARQ transmissions received onthe reverse link) and generates signaling symbols. A modulator (MOD)1212 performs modulation (e.g., for OFDM, SC-FDM, CDMA, etc.) on thedata symbols, signaling symbols, and pilot symbols and provides outputchips. A transmitter (TMTR) 1214 conditions (e.g., converts to analog,filters, amplifies, and upconverts) the output chips and generates aforward link signal, which is transmitted via an antenna 1216.

At terminal 1250, an antenna 1252 receives forward link signals frombase station 1200 and possibly other base stations. A receiver (RCVR)1254 processes (e.g., amplifies, downconverts, filters, and digitizes)the received signal from antenna 1252 and provides received samples. Ademodulator (DEMOD) 1256 performs demodulation (e.g., for OFDM, SC-FDM,CDMA, etc.) on the received samples and provides received symbols. Areceive (RX) data and signaling processor 1258 processes (e.g., symboldemaps, deinterleaves, and decodes) the received symbols and providesdecoded data for terminal 1250 to a data sink 1260 and recoveredsignaling to a controller/processor 1270. Processor 1260 may performdecoding for one or more SOP hypothesis for each packet sent on theforward link to terminal 1250.

For reverse link transmission, at terminal 1250, a TX data and signalingprocessor 1264 receives packets of data from a data source 1262 andgenerates a set of data symbols for each packet. Processor 1210 mayprovide a different subset of data symbols for each transmission of agiven packet sent with H-ARQ. Processor 1264 also generates signalingsymbols for signaling to be sent to base station 1200, e.g., ACKs/NAKsfor H-ARQ transmissions received on the forward link. A modulator 1266performs modulation on the data symbols, signaling symbols, and pilotsymbols and provides output chips. A transmitter 1268 conditions theoutput chips and generates a reverse link signal, which is transmittedvia antenna 1252.

At base station 1200, reverse link signals from terminal 1250 and otherterminals are received by antenna 1216, conditioned and digitized by areceiver 1220, demodulated by a demodulator 1222, and processed by an RXdata and signaling processor 1224 to recover packet data and signalingsent by terminal 1250 and other terminals. Processor 1224 may performdecoding for one or more SOP hypothesis for each packet sent on thereverse link to base station 1200. Processors 1224 and 1258 may eachimplement process 800 in FIG. 8, process 1000 in FIG. 10, and/or otherprocesses for the techniques described herein.

Controllers/processors 1230 and 1270 direct the operation at basestation 1200 and terminal 1250, respectively. Processors 1230 and 1270may each implement process 1000 in FIG. 10 and/or other processes forthe techniques described herein. Memories 1232 and 1272 store programcodes and data for base station 1200 and terminal 1250, respectively. Acommunication (Comm) unit 1236 allows base station 1200 to communicatewith other network entities via a backhaul. A scheduler 1234 schedulesthe terminals being served by base station 1200 for transmission on theforward and reverse links.

FIG. 13 shows a block diagram of a design of an RX data processor 1300,which may be part of processor 1224 and/or processor 1258 in FIG. 12.Within processor 1300, the received symbols in each frame are providedto a traffic detector 1310 and a symbol buffer 1320. Traffic detector1310 performs traffic detection in each frame based on the receivedsymbols for that frame and provides a “pass” or “fail” indication. A SOPdetector 1312 receives the pass/fail indications from traffic detector1310 and decoding results from a decoder 1330 and provides a SOPdecision for each packet. The decoding result for a decoding attempt maybe given by a cyclic redundancy check (CRC) pass if a packet is decodedcorrectly or a CRC failure if the packet is decoded in error.

Symbol buffer 1320 stores the received symbols and provides thesereceived symbols at appropriate time. An LLR computation unit 1322computes LLRs for code bits in the received transmission in each framebased on the received symbols and a modulation scheme used for thatframe. Unit 1322 may be controlled by a control signal from an LLRcontrol unit 1332. An input buffer selector 1324 receives the LLRs fromunit 1322 and provides these LLRs to one of N buffers 1326 a through1326 n based on a select signal from LLR control unit 1332. Each buffer1326 stores LLRs for one SOP hypothesis for one packet at any givenmoment. Each buffer 1326 may store and/or combine LLRs whenever a newtransmission is received for the current packet. An output bufferselector 1328 receives the LLRs from one of N buffers 1326 a through1326 n for each decoding attempt and provides these LLRs to decoder1330. A decoder control unit 1334 provides a select signal that selectsone of the N buffers 1326 for each decoding attempt. Decoder 1330decodes the LLRs from selector 1328 for each decoding attempt andprovides the decoding status (e.g., CRC pass or fail) for that decodingattempt as well as a decoded packet if the decoding is successful.

LLR control unit 1332 receives Reset and Stop commands (e.g., fromcontroller 1230 or 1270) and the decoding results (e.g., CRC) fromdecoder 1330 and generates the control signal for unit 1322 and theselect signal for selector 1324. Decoder control unit 1334 receives theSOP decisions (SOP) from SOP detector 1312 and the decoding results(CRC) from decoder 1330 and generates the select signal for selector1328 and the control signal for decoder 1330.

FIG. 14 shows a state diagram 1400 that may be used for LLR computationunit 1322 and decoder 1330 in FIG. 13. In an LLR off and decoder offstate 1410, LLR computation and decoding are not performed. In an LLR onand decoder off state 1420, LLR computation is performed on receivedsymbols, and decoding is not performed. In an LLR on and decoding onstate 1430, LLR computation is performed on received symbols, anddecoding is performed on LLRs. State 1410 may be entered at power up andalso upon receiving a Stop command while in either state 1420 or 1430.State 1420 may be entered upon receiving a Reset command while in state1410 and also upon receiving a CRC pass for a packet while in state1430. State 1430 may be entered upon making a SOP decision for a packet.

Table 1 lists some example operations that may be performed in responseto each of the commands shown in FIG. 14.

TABLE 1 Command Operations Reset Clear current SOP hypotheses, Configuresliding SOP window size and alignment, Start forming new SOP hypotheseswithin SOP window in next frame, and Allow SOP window to slide withincoming frame. Stop Clear current SOP hypotheses, and Do not form anynew SOP hypothesis. SOP Maintain SOP window at current frame, Continueforming SOP hypotheses within SOP window, and Start decoding for SOPhypotheses within SOP window. CRC Stop decoding for the current packet,Clear current SOP hypotheses, Start forming new SOP hypotheses withinSOP window in next frame, and Allow SOP window to slide with incomingframe.

FIG. 6A shows an example of when the commands in Table 1 may begenerated for an example sequence of transmissions. The Reset commandmay be issued prior to the first received transmission and may be usedto initialize the sliding SOP window. The SOP command may be issuedwhenever a SOP decision is made for a packet, e.g., based on the trafficdetection for the current frame and the decoding results for the priorframe. The CRC command may be issued whenever a packet is decodedcorrectly. The Stop command may be issued when data transmission isterminated or paused.

The traffic detection and packet decoding for H-ARQ may be performed bymultiple layers such as a Medium Access Control (MAC) layer and aphysical (PHY) layer. The MAC layer may maintain state diagram 1400,generate Reset, Stop, SOP, and CRC commands, and issue these commands tothe PHY layer. The PHY layer may perform LLR computation and packetdecoding based on the commands received from the MAC layer. The PHYlayer may also keep track of the N buffers, assign buffers to new SOPhypotheses, select SOP hypothesis for decoding, etc. The set of commandsin Table 1 may provide a simple interface between the MAC and PHYlayers, which may simplify the design and/or the operation of theselayers, allow for flexible MAC design, and allow the MAC to control PHYoperation with few commands. Other commands may also be used to supportLLR computation and packet decoding for H-ARQ with sliding SOP windowand/or rotating buffers.

The techniques described herein may provide various advantages. Thetechniques may be able to efficiently handle multiple SOP hypotheses aswell as delayed and time changing SOP hypotheses. The sliding SOP windowmay be able to handle delayed SOP hypotheses. The rotating buffers maybe able to handle multiple SOP hypotheses. The simple interface may beable to handle changing SOP hypotheses. The techniques may reducecomplexity while supporting flexible MAC logic for reliable H-ARQ packetdecoding. The techniques may also be able to streamline SOP hypothesesprocessing to reduce data buffering and decoding.

The techniques described herein may be implemented by various means. Forexample, these techniques may be implemented in hardware, firmware,software, or a combination thereof. For a hardware implementation, theprocessing units used to implement the techniques may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, electronic devices, other electronic units designed toperform the functions described herein, a computer, or a combinationthereof.

For a firmware and/or software implementation, the techniques may beimplemented with modules (e.g., procedures, functions, etc.) thatperform the functions described herein. The firmware and/or softwarecodes may be stored in a memory (e.g., memory 1232 or 1272 in FIG. 12)and executed by a processor (e.g., processor 1230 or 1270). The memorymay be implemented within the processor or external to the processor.

The previous description of the disclosure is provided to enable anyperson skilled in the art to make or use the disclosure. Variousmodifications to the disclosure will be readily apparent to thoseskilled in the art, and the generic principles defined herein may beapplied to other variations without departing from the spirit or scopeof the disclosure. Thus, the disclosure is not intended to be limited tothe examples described herein but is to be accorded the widest scopeconsistent with the principles and novel features disclosed herein.

1. An apparatus for H-ARQ transmission comprising: at least oneprocessor configured to: receive a plurality of transmissions for apacket; determine a sliding window indicative of at least one start ofpacket (SOP) hypothesis for the packet, wherein the SOP hypothesis isdetermined for the packet based on an SOP decision and each SOPhypothesis may correspond to a different frame hypothesized to be theSOP of the packet and wherein the sliding window keeps track of SOPhypotheses for the packet; and decode the plurality of receivedtransmissions based on the at least one SOP hypothesis for the packet; amemory coupled to the at least one processor; and wherein the at leastone processor is configured to initialize said sliding window at anearliest received transmission among the plurality of receivedtransmissions to move the sliding window forward for each subsequentreceived transmission until a SOP decision is made, and to maintain thesliding window at a received transmission where the SOP decision ismade.
 2. The apparatus of claim 1, wherein the plurality of receivedtransmissions comprise at least one transmission without packet data andat least one transmission with packet data.
 3. The apparatus of claim 1,wherein the at least one processor is configured to detect whether eachof the plurality of received transmissions carry packet data or nopacket data, and to determine the sliding window based on detectionresults for the plurality of received transmissions.
 4. The apparatus ofclaim 1, wherein the at least one processor is configured to determine afirst received transmission with packet data among the plurality ofreceived transmissions, and to center the sliding window at the firstreceived transmission with packet data.
 5. The apparatus of claim 1,wherein the at least one processor is configured to center the slidingwindow at a designated received transmission among the plurality ofreceived transmissions.
 6. The apparatus of claim 1, wherein theplurality of transmissions are received in a plurality of frames, onetransmission in each frame, and wherein each SOP hypothesis correspondsto a different frame hypothesized to be the SOP of the packet.
 7. Theapparatus of claim 6, wherein for each frame in the plurality of framesthe at least one processor is configured to determine one or more SOPhypotheses applicable for the frame and to perform decoding for one SOPhypothesis at a time among the one or more SOP hypotheses.
 8. Theapparatus of claim 6, wherein for each frame in the plurality of framesthe at least one processor is configured to determine one or more SOPhypotheses applicable for the frame and to perform decoding for each SOPhypothesis based on transmissions received for the SOP hypothesis in theframe or earlier.
 9. The apparatus of claim 1, wherein the memory isconfigured to store transmissions received prior to a first receivedtransmission with detected packet data, and wherein the at least oneprocessor is configured to start decoding when the first receivedtransmission with packet data is detected.
 10. The apparatus of claim 1,wherein the sliding window includes up to L SOP hypotheses for up to Lreceived transmissions with no detected packet data prior to a firstreceived transmission with detected packet data, where L is an integervalue.
 11. The apparatus of claim 1, wherein the sliding window includesno SOP hypotheses prior to a first received transmission with detectedpacket data if a transmission received immediately prior to the firstreceived transmission with detected packet data is for another packetthat is correctly decoded.
 12. The apparatus of claim 1, wherein thesliding window includes up to R SOP hypotheses for up to R receivedtransmissions after a first received transmission with detected packetdata, where R is an integer value.
 13. The apparatus of claim 1, whereinthe sliding window includes up to three SOP hypotheses for the packetand up to one SOP hypothesis for up to one received transmission with nodetected packet data prior to a first received transmission withdetected packet data.
 14. The apparatus of claim 1, wherein the at leastone processor is configured to generate a reset command to clear priorSOP hypotheses, to start forming new SOP hypotheses for the packet, toallow the sliding window to move forward with each receivedtransmission, or a combination thereof.
 15. The apparatus of claim 1,wherein the at least one processor is configured to generate a stopcommand to clear current SOP hypotheses, to stop forming new SOPhypotheses, or both.
 16. The apparatus of claim 1, wherein the at leastone processor is configured to generate a SOP command to maintain thesliding window at current location, to continue forming SOP hypotheseswithin the sliding window for the packet, to start decoding for SOPhypotheses within the sliding window, or a combination thereof.
 17. Theapparatus of claim 1, wherein after successful decoding of the packetthe at least one processor is configured to generate a command to stopdecoding for the at least one SOP hypothesis for the packet, to clearthe at least one SOP hypothesis, to start forming new SOP hypotheses fora next packet, to allow the sliding window to slide with each subsequentreceived transmission, or a combination thereof.
 18. A method of packetdecoding for H-ARQ transmissions comprising: receiving a plurality oftransmissions for a packet; determining a sliding window indicative ofat least one start of packet (SOP) hypothesis for the packet, whereinthe SOP hypothesis is determined for the packet based on an SOP decisionand each SOP hypothesis may correspond to a different frame hypothesizedto be the SOP of the packet and wherein the sliding window keeps trackof SOP hypotheses for the packet; decoding the plurality of receivedtransmissions based on the at least one SOP hypothesis for the packet;and wherein determining the sliding window comprises: initializing thesliding window at an earliest received transmission among the pluralityof received transmissions, moving the sliding window forward for eachsubsequent received transmission with no detected packet data, andmaintaining the sliding window at a first received transmission withdetected packet data.
 19. The method of claim 18, wherein the pluralityof transmissions are received in a plurality of frames, and wherein thedecoding comprises, for each frame in the plurality of frames,determining one or more SOP hypotheses applicable for the frame, andperforming decoding for one SOP hypothesis at a time among the one ormore SOP hypotheses.
 20. The method of claim 18, further comprising:storing transmissions received prior to a first received transmissionwith detected packet data; and starting decoding when the first receivedtransmission with packet data is detected.
 21. The method of claim 18,further comprising: assigning at least one of a plurality of buffers tothe at least one SOP hypothesis for the packet, one buffer for each SOPhypothesis; and storing at least one received transmission for each SOPhypothesis in a buffer assigned to the SOP hypothesis.
 22. The method ofclaim 21, further comprising: selecting the plurality of buffers forassignment to SOP hypotheses in a predetermined order.
 23. An apparatuscomprising: means for receiving a plurality of transmissions for apacket; means for determining a sliding window indicative of at leastone start of packet (SOP) hypothesis for the packet, wherein the SOPhypothesis is determined for the packet based on an SOP decision andeach SOP hypothesis may correspond to a different frame hypothesized tobe the SOP of the packet and wherein the sliding window keeps track ofSOP hypotheses for the packet; means for decoding the plurality ofreceived transmissions based on the at least one SOP hypothesis for thepacket; and means for initializing the sliding window at an earliestreceived transmission among the plurality of received transmissions,means for moving the sliding window forward for each subsequent receivedtransmission with no detected packet data, and means for maintaining thesliding window at a first received transmission with detected packetdata.
 24. The apparatus of claim 23, wherein the plurality oftransmissions are received in a plurality of frames, and wherein themeans for decoding comprises, for each frame in the plurality of frames,means for determining one or more SOP hypotheses applicable for theframe, and means for performing decoding for one SOP hypothesis at atime among the one or more SOP hypotheses.
 25. The apparatus of claim23, further comprising: means for storing transmissions received priorto a first received transmission with detected packet data; and meansfor starting decoding when the first received transmission with packetdata is detected.
 26. The apparatus of claim 23 further comprising:means for assigning at least one of a plurality of buffers to the atleast one SOP hypothesis for the packet, one buffer for each SOPhypothesis; and means for storing at least one received transmission foreach SOP hypothesis in a buffer assigned to the SOP hypothesis.
 27. Anon-transitory processor-readable medium including instructions storedthereon, comprising: a first instruction set for receiving a pluralityof transmissions for a packet; a second instruction set for determininga sliding window indicative of at least one start of packet (SOP)hypothesis for the packet, wherein the SOP hypothesis is determined forthe packet based on an SOP decision and each SOP hypothesis maycorrespond to a different frame hypothesized to be the SOP of the packetand wherein the sliding window keeps track of SOP hypotheses for thepacket; a third instruction set for decoding the plurality of receivedtransmissions based on the at least one SOP hypothesis for the packet;wherein the second instruction set for determining the sliding windowcomprises: a fourth instruction set for initializing the sliding windowat an earliest received transmission among the plurality of receivedtransmissions, a fifth instruction set for moving the sliding windowforward for each subsequent received transmission with no detectedpacket data, and a sixth instruction set for maintaining the slidingwindow at a first received transmission with detected packet data. 28.An apparatus comprising: at least one processor configured to receive aplurality of transmissions in a plurality of frames for a packet;determine a sliding window indicative of at least one start of packet(SOP) hypothesis for the packet, wherein the SOP hypothesis isdetermined for the packet based on an SOP decision and each SOPhypothesis may correspond to a different frame hypothesized to be theSOP of the packet and wherein the sliding window keeps track of SOPhypotheses for the packet, to make a start of packet (SOP) decision forthe packet, the SOP decision indicating one of the plurality of framesas a SOP of the packet, to determine at least one SOP hypothesis for thepacket based on the SOP decision, and to decode the plurality ofreceived transmissions based on the at least one SOP hypothesis for thepacket; a memory coupled to the at least one processor; and wherein theat least one processor is configured to initialize said sliding windowat an earliest received transmission among the received transmissions tomove the sliding window forward for each subsequent receivedtransmission until a SOP decision is made, and to maintain the slidingwindow at a received transmission where the SOP decision is made. 29.The apparatus of claim 28, wherein the at least one processor isconfigured to detect whether each of the plurality of receivedtransmissions carry packet data or no packet data, to identify a firstreceived transmission with detected packet data among the plurality ofreceived transmissions, and to provide a frame with the first receivedtransmission with detected packet data as the SOP decision.
 30. Theapparatus of claim 28, wherein the at least one SOP hypothesis comprisesone SOP hypothesis for the SOP decision, up to one SOP hypothesis for upto one frame prior to the SOP decision, and up to one SOP hypothesis forup to one frame after the SOP decision.
 31. The apparatus of claim 28,further comprising: a plurality of buffers; and at least one processorcoupled to the plurality of buffers and configured to determine at leastone start of packet (SOP) hypothesis for a packet, to assign at leastone of the plurality of buffers to the at least one SOP hypothesis forthe packet, one buffer for each SOP hypothesis, each buffer storing atleast received transmission for the SOP hypothesis to which the bufferis assigned, wherein the SOP hypothesis is determined for the packetbased on an SOP decision and each SOP hypothesis may correspond to adifferent frame hypothesized to be the SOP of the packet. one
 32. Theapparatus of claim 31, wherein the at least one processor is configuredto select the plurality of buffers for assignment to SOP hypotheses in apredetermined order.
 33. The apparatus of claim 31, wherein the at leastone processor is configured to determine a SOP hypothesis for a secondpacket, and to assign a next buffer to the SOP hypothesis for the secondpacket, the next buffer being after the at least one buffer among theplurality of buffers.
 34. The apparatus of claim 31, wherein the numberof buffers is equal to a maximum number of possible SOP hypotheses forthe packet.
 35. The apparatus of claim 31, wherein each buffer isconfigured to store log-likelihood ratios (LLRs) for the at least onereceived transmission for the SOP hypothesis to which the buffer isassigned.